A good quality double row camshaft gear produced in the UK.  This is the standard replacement item for the Triumph TR6, but it can also replace the single row gears found on the Spitfires, GT6s and TR250s as a durability improvement (you'll need to change your crank gear, timing chain and tensioner all to double row components as well).
July 2011: In the past year inexpensive hardened cam and crank gears have come onto the market.  These gears are made in India, and while they are indeed hardened, the quality of the gear teeth is sub-standard.  Additional, for crank gears, the keyway is machined inaccurately.  The result is a sloppy, yet hard, component that will give imprecise timing in addition to increased timing chain wear.